1. Field of the Invention
The present invention relates to a precursor sol of aluminum oxide and a method for producing an antireflective optical member with the precursor sol of aluminum oxide. In particular, the present invention relates to a method for producing an optical member configured to stably deliver high antireflection performance in a wide wavelength range including the visible region.
2. Description of the Related Art
Antireflective structures with periodic microstructures which has repetition periods each equal to or lower than a wavelength in the visible region and which has appropriate pitches and heights are known to deliver excellent antireflection performance in a wide wavelength.
An example of a known method for forming a periodic microstructure is coating in which a film including fine particles dispersed therein is formed by coating, the fine particles each having a particle size equal to or lower than a wavelength in the visible region. U.S. Pat. No. 4,013,465 discloses that a method for forming a periodic microstructure by patterning with micromachining equipment, e.g., an electron beam lithography system, a laser interference exposure apparatus, a semiconductor exposure apparatus, or an etching apparatus, enables us to control the pitch and height of the periodic microstructure and to form a periodic microstructure having excellent antireflection performance.
It is known that as another method, boehmite, which is aluminum oxide hydroxide, is grown on a base to provide an antireflection effect. For example, Japanese Patent Laid-Open No. 9-202649 discloses that an aluminum (aluminum oxide) film formed by a liquid-phase method (sol-gel method) is subjected to steam treatment or hot-water immersion treatment to modify its surface into a boehmite surface having a periodic microstructure, thereby affording an antireflection coating.
It is known that in the method for forming an antireflection coating with a periodic microstructure composed of boehmite, the resulting film has extremely low reflectivity at normal incidence and high antireflection performance. However, a constant height and a constant period of the periodic microstructure are required to maintain the high antireflection performance. In the case of forming a periodic microstructure by the vacuum film formation method, it is difficult to control the thickness of a film formed on a curved surface or a wide surface. Meanwhile, in the case of employing the method in which an aluminum oxide film is formed by a liquid-phase method (sol-gel method) and then subjected to hot-water immersion treatment, a periodic microstructure can be formed on a surface having any shape. However, a precursor sol of aluminum oxide prepared by hydrolysis of an aluminum compound is not always stable as a coating material. It is thus difficult to form an antireflection coating with uniform reflectivity by uniformly applying the precursor sol to a base and then performing the hot-water treatment.
In the liquid-phase method (sol-gel method) in which an antireflection coating is formed with the precursor sol of aluminum oxide, it is desirable to provide a precursor sol of aluminum oxide, the precursor sol being stable as a coating material without causing aggregation, and a simple method for producing a high-performance uniform antireflection coating with the aluminum-oxide-precursor sol.